This invention relates to systems for checking the validity of coins used to operate vending machines and other apparatus in order to detect and reject counterfeits.
It is a common experience for the proprietors of vending machines and coin operated systems of various types, such as amusement games, washing machines and dryers, to suffer the loss of revenue because certain patrons use slugs or other invalid coinage. In order to deal with this problem, a great number of systems have been designed to detect and reject spurious coinage. These systems generally are based on one or more electrical characteristics of a valid coin. U.S. Pat. No. 3,599,711 to Hinterstocker, for example, appears to rely upon the known conductivity of valid coins. In that system, a standard coin and a test coin are used to affect the coupling between a primary coil and two pickup coils and the results compared. If the conductivity of the test coin matches that of the standard coin, then a null reading should be obtained when the pickup coils are connected in series opposition so that they are "bucking" one another. The system described in U.S. Pat. No. 3,596,744 to Chesnokov uses two coils wound around a coin chute, one coil being a primary and the other a secondary. The coils are spaced sufficiently far apart so that they are not inductively coupled; yet, they are sufficiently close so that they may be inductively coupled when a coin, having a ferromagnetic phase, is passing. When that happens, an EMF is induced in the secondary coil and its magnitude will be dependent upon the ferromagnetic properties of the coin. U.S. Pat. No. 4,151,904 to Lavasseur, et al., also has a coil wrapped around a chute, which coil is driven by an oscillator at a known frequency. This system operates by detecting the change in frequency which occurs on the passing of a coin. The frequency change is related to the conductive properties of the coin under test and should fall within predetermined limits for known valid coins.
One of the desirable attributes of a coin validation system based upon a determination of its ferromagnetic phase is that it is difficult to fabricate spurious coinage that will be accepted as valid by the system. This is because the ferromagnetic content or phase of a coin can be held within rather close tolerances during its manufacture. Alloys having a ferromagnetic phase have what has been called an "intrinsic induction" which depends only on the temperature and chemical composition of the ferromagnetic phase. This intrinsic induction determines the permeability of the material and hence, the degree to which it can change the inductance of a coil when used as the core thereof. For materials deriving their ferromagnetism from a single element, such as iron, nickel or cobalt, processing such as sintering, casting, rolling, coining, etc., do not affect the intrinsic induction since they do not modify the ferromagnetic atomic structure.
The intrinsic induction in a composite material having iron as its only ferromagnetic phase varies linearly in proportion to the iron concentration. For example, compacts of brass and iron powders can be made with the iron varying from 0% to 60%. Thus, it is possible to "tag" coins by preparing them with a prescribed amount of iron as the single ferromagnetic substance therein. Indeed, one can prepare coins having different percentages of iron therein so that they can be used on some random basis. For example, in an amusement part, the operator might issue coins having a first concentration of iron for a certain time period during which such coins would be acceptable. Later, the concentration could be changed to thwart those who may have been able to successfully duplicate the initial issue of coins.
In order to take advantage of magnetic tagging as a means of coin validation, one must be able to accurately discern differences in the ferromagnetic properties of coins. The degree of accuracy required does not appear to be available in any prior art device of which I am aware. I have discovered, however, a system in which the necessary degree of sensitivity can be achieved. Moreover, the basic sensing unit and associated circuitry is relatively simple and inexpensive to manufacture. Because of its simplicity and its use of well-known materials and components, it is extremely reliable and should provide relatively trouble-free operation.
My invention centers in the provision of an induction coil system wherein a test coin is compared with a standard coin. The key to my invention lies in the configuration and arrangement of secondary or pickup coils within a magnetic field and the provision of structures which will result in having the planes of the coins involved generally parallel to the flux lines of the magnetic field in which the pickup coils are placed. Such an arrangement minimizes the demagnetizing effects normally experienced when permeable materials are placed in a magnetic field and permits maximum magnetization thereof. This is quite important because the resultant changes in the EMF's produced by the pickup coils are relatively small and, therefore, must be maximized.
Briefly stated, the preferred embodiment of this invention involves the winding of a pickup coil around the coin chute of the vending machine or other apparatus which one desires to monitor. A substantially identical pickup coil is wrapped around a similar chute placed alongside of the coin chute. Then both chutes are enveloped by the windings of a primary coil which will serve to induce voltages in the pickup coils. The pickup coils are connected in a series opposed relationship with one another and their combined output fed to appropriate analyzing circuitry. Assuming the coils to be balanced so as to produce a null reading, one determines the validity of coins by using a standard coin as the core for one pickup coil while a coin to be tested is used as the core for the other pickup coil. Assuming the two similarly shaped and sized coins are made of materials having the same ferromagnetic properties, the combined output of the coils should continue to be null.
Otherwise stated, the method of this invention is to place a pair of identical coils in a substantially uniform magnetic field and then to use a standard coin of known ferromagnetic properties and a coin to be tested as core materials for the respective pickup coils. Assuming a match in the ferromagnetic composition of the two coins, as well as a match in shape and size, one should obtain a null reading for coils connected in a "bucking" relationship.